In exchange 2013 how can I prevent ALL ndr's from being sent?  Spammers are using the NDR's to send out spam.  I've already configured the machine to reject mail to anyone not in our mailbox.

Hi
   You can just prevent NDR remote domain.
   http://technet.microsoft.com/en-us/library/jj649325(v=exchg.150).aspx

Probably You can use:

Set-RemoteDomain * -NDREnabled $False.

This will disable sending NDR for default Remote Domain. Thanks Terence Yu for idea!

Thanks a lot. Perfectly solution. 

This has helped with an issue I completely ignored

Cheers